The United States government stringently regulates motor vehicle emission levels for pollutants such as carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOx). Engine performance and pollutant emissions depend upon the air-fuel mixture supplied to an engine.
A fuel metering system, that monitors oxygen levels in the exhaust gases, controls the quantity of fuel contained in the air-fuel mixture. An oxygen sensor, located between the motor vehicle engine and the catalytic converter in the engine exhaust system, provides precision feedback to the metering system enabling it to make immediate adjustments to the air-fuel mixture. Accurate feedback from the oxygen sensor to the fuel metering system is essential for proper regulation of the level of pollutants in motor vehicle exhaust gases. Such accuracy, in turn, requires a properly functioning oxygen sensor.
Due to the proximity of the oxygen sensor to the vehicle engine, exhaust gases contacting the sensor are very hot and chemically active—conditions which cause aging of the sensor. Hence, vehicles have used a variety of methods of attempting to ascertain whether a sensor has aged to the point of requiring replacement.
Known diagnostic routines for monitoring performance of exhaust stream oxygen sensors are “intrusive”—i.e., such routines may interfere with, or intrude upon, an engine control module's normal fuel metering functions for minimizing undesirable exhaust emissions. Such conventional diagnostics likewise intrude upon a control module's capability to optimize a variety of drivability characteristics of the vehicle.
Hence, there is seen to be a need for a non-intrusive diagnostic method for judging whether an exhaust gas oxygen sensor requires replacement.